The present invention relates a printing or recording apparatus employing electrophotography, such as printer, copying machine and facsimile transmitter. More particularly, it relates to a fixing apparatus wherein a toner image formed on the surface of a recording medium is fixed to the recording medium.
A conventional fixing apparatus has, at least, a heat roller. By the way, in this specification, the heat roller and a backup roller will be sometimes abbreviated as "HR" and "BR", respectively, for the sake of brevity. Both the rollers HR and BR shall be called a "pair of fixing rollers". Besides, any one of the HR and BR will be called a "fixing roller" in some cases. In the fixing apparatus, toner sometimes adheres to the fixing roller in case of fixing a toner image. This phenomenon is termed "offset", while the toner adherent to the fixing roller is termed "offset toner". In a case where there is a large amount of offset toner, there arises the problem that the offset toner remigrates onto a recording medium and becomes indistinguishable from recorded information thereby causing misprints to occur.
FIG. 1 is a diagram showing the known appearance characteristic of the offset toner in dependence of temperature. Numeral 19 designates a low temperature region, numeral 20 a high temperature region, and numeral 21 a non-offset band. The abscissa represents the temperature of the fixing roller, while the ordinate axis represents the quantity of the offset toner. The quantities of the offset toner are large in the low temperature region 19 and the high temperature region 20. The offset occurring in the low temperature region 19 is termed "low temperature offset", while the offset occurring in the high temperature region 20 is termed "high temperature offset". The region between the low temperature region 19 and the high temperature region 20 is termed the "non-offset band 21", in which the quantity of the offset toner is small. In the fixing apparatus, the temperature of the fixing roller is usually set within the non-offset band 21 to decrease the quantity of the offset toner to the utmost. However, in the conventional device, even when the temperature of the fixing roller is set within the non-offset band 21, some offset takes place, offset toner is gradually accumulated and the above-mentioned misprint problem arises.
Japanese Patent Application Publication No. 15072/1990 discloses is by way of example, a cleaning device using bristles by which the bristles are endowed with a release agent, whereby the caught offset toner adherent to the bristles is separated from these bristles. However, this causes the problem that the offset toner separated from the bristles pollutes the interior of the fixing apparatus.